It does because when a keyboard is connected to the USB port, the keyboard is being polled so that when a keypress is sensed, an interrupt request is sent so that your keypress registers just as you expect it to. So when you press a key, it goes "I just sensed a keystroke! Send an interrupt request immediately!"

But when a keyboard is connected through PS/2 (even through a USB to PS/2 adapter, if the keyboard works through one of those), it's not being polled at all. Instead, every single keystroke sends an interrupt request.

So polling consumes CPU cycles, constantly monitoring the keyboard for any keystrokes. It's nothing to shake a stick at, but it's still happening. Plus, it's possible for a USB keyboard to be slightly interrupted by other devices connected to one of the other USB ports. But when it's connected to the PS/2 port, it's perfectly isolated and it gets all the attention it wants every single time it wants it.

Fortunately, it doesn't make a noticeable difference on a system like yours or mine, so it's not really a valid concern. But for purists, it's almost fun to prefer that the keyboard is connected to the PS/2 port. That way, the only thing that's being polled is the mouse, and that's actually preferred for superior mouse performance.

But ultimately, the main benefit these days is N Key Rollover with mechanical keyboards that specifically offer it. But even that's slowly changing.Edited by TwoCables - 3/10/11 at 10:16pm